In the manufacturing of automotive and appliance grade steel sheet and strip, red hot steel slabs are hot reduced in one or several steps into a steel band of about 1/4" thickness. In this operation, a heavy mill scale of iron oxides forms on the steel which has to be removed by acid pickling. This operation leads to considerable losses of metal from the surface. For further reduction, the surface area is increased manyfold and hot rolling becomes impractical. Therefore, the hot band is reduced further by cold rolling. For this step, it is essential to employ a rolling oil to reduce friction and to facilitate a larger cross sectional reduction. Cold rolling of carbon steel introduces work hardening, which can be considerable in modern multi-stand mills. Too much hardening renders the sheet metal unsuitable for further forming operations, be it tight bending, deep drawing, punching or further gauge reduction. To remove the excess hardness, the steel has to be annealed by heating it for about 10 to 12 hours to temperatures between 1000.degree. and 1400.degree. F. in a reducing atmosphere.
After the cold rolling step, the rolling oil is usually left on the steel, partly for rust protection for the time in storage prior to anneal, but mainly because of the expense in removing it. Cleaning steel is costly because it requires extra process equipment. It cannot be done in-line with the rolling operation because cleaning requires considerably more time than rolling. Occasionally, a mill cleaner is applied in line but because of the high speed, only marginal cleaning is achieved, i.e. an oil film from the rolling oil remains on the steel. Usually the steel sheet, rolled up in large coils, is annealed with an oily surface.
Modern rolling oils are mixtures of many different compounds. They are normally applied from a water based emulsion in concentrations from 0.5 to 15%. The main ingredients are mineral oils (i.e. hydrocarbons), fatty oils and greases (i.e. polyalcohol esters of fatty acids), and fatty amides. Other additives are present which are necessary for a satisfactory performance, such as detergents, emulsifiers, fungicides, bactericides, antioxidants and high pressure lubricant additives. None of these compounds is stable at a temperature above 1000.degree. F. In air, they simply burn off. In the reducing atmosphere used in the annealing chamber, the rolling oil decomposes. Some of it is distilled off, but enough of it breaks down into gaseous products and higher weight materials which finally end up as a carbonaceous layer on the steel surface called carbon smut. The amount of carbon smut deposit and its structure depends on many parameters, i.e. composition of the rolling oil, type of steel, rate of heatup, duration of heat cycle while annealing, peak temperature and type of annealing gas. In most cases, the presence of carbon smut on the steel surface is detrimental for later surface treatments.
Most of the steel sheet is used for manufacturing products that will later receive an organic finish such as a paint or polymer coating. To get a useful life out of a paint coat, a proper surface preparation is essential. This mostly requires a conversion coating based on phosphates, which acts as an adhesion promoter and corrosion stopper in case of physical damage to the paint. Phosphate coatings are applied from mildly acidic water based solutions. Although some carbon smut deposits on annealed steel are loose and can be washed off, many are very dense and form a tenaceous skin that resists industrial cleaners. Even the acidity in the phosphating solution is not sufficient to penetrate the dense carbon deposits of smutted steel and very poor phosphate coatings are obtained. The only certain ways to remove the carbon smut sufficiently are either mechanically by surface grinding or by shot blasting, or chemically by strong acid pickling. Both such types of preparation are expensive and time consuming and often not possible to integrate in existing production lines.